1. Field of the Invention
The present invention relates to liquid crystal panels, which have collecting micro-lenses opposing pixel electrodes driving liquid crystal pixels so as to achieve higher luminance, and projection type display devices using such liquid crystal panels.
2. Description of the Related Art
Recently, liquid crystal projectors and liquid crystal projection TVs, in which an image on a liquid crystal panel is projected onto a screen by a magnifying optical projection system utilizing a liquid crystal panel as an optical switching element, have been popularly developed. These apparatuses are advantageous in that they are thin and lightweight, have sharp images, are not affected by earth's magnetic field, and do not require registration adjustment.
Such liquid crystal display devices are classified into single-panel systems composed of a liquid crystal panel having color filters for three colors, i. e., B (blue), R (red), and G (green), and three-panel systems having monochrome liquid crystal panels for the B, R, and G optical paths. According to the single-panel systems, a compact and lightweight liquid crystal device can be readily formed at a lower cost because of its simple structure. However, since the color filters absorb a large amount of light, it is difficult to achieve higher luminance and efficient cooling.
To solve such problems, for example, Japanese Patent Laid-Open No. 4-60538 (hereinafter referred to as "document (i)") and "ASIA DISPLAY '95, p 887" (hereinafter referred to as "document (ii)") disclose color liquid crystal display devices in which collecting micro-lenses are arranged as follows: one collecting micro-lens opposes every three pixel electrodes driving liquid crystal pixels, and light beams of three colors, i. e., B, R, and G, enter each of the micro-lenses from mutually different directions so as to be collected, and the resultant emerging light beam of each color enters a pixel electrode corresponding to the color of the emerging light beam. In this color liquid crystal display device, light beams which would normally enter the regions between the pixels (the matrix of opaque regions in which thin-film transistors (TFTs) are formed as pixel driving elements) can be effectively utilized so that the effective aperture ratio increases, thereby achieving a higher luminance.
According to such color liquid crystal display devices, the focal points of the micro-lenses opposing the pixel electrodes are positioned near the corresponding pixel portions. In other words, collimated light entering the micro-lens is collected to focus near the pixel portion, and then, diverges again.
Although data projectors and rear projection TVs based on the liquid crystal projection system have already been put into practical use, it is supposed that with the development of multi-media, these devices are required to display computer and AV (audio.multidot.video) images on the same panel at a resolution as high as that of high-definition televisions. In such a case, the optical system including the liquid crystal display elements must have higher resolution, higher image quality, and higher luminance as compared with conventional optical systems. For example, a liquid crystal display panel employed in presently used rear-projection TVs uses TFTs made of amorphous silicon (a-Si), and the total number of pixels is approximately 1,300,000 or less in a picture size of 3 to 5 inches. However, to achieve thinner and lighter devices according to the liquid crystal projection system, it is necessary to increase the pixel density to approximately 1,500,000 to 2,000,000 pixels in a picture size of 2 inches. Moreover, such compact high-resolution LCD panels, including their optical system, are advantageous in reducing prices. Thus, a further increase in consumer demand is expected in the future. Concerning process techniques, it is supposed that high-temperature polysilicon (polycrystalline silicon) TFT techniques or low-temperature polysilicon TFT techniques become important for producing such high-resolution liquid crystal panels.
As mentioned above, there is a greater necessity to reduce the area of pixel portions in liquid crystal projectors to achieve higher resolution. Thus, from now on, the TFTs, as the pixel driving elements, are required to be formed from polysilicon instead of amorphous silicon. This is because in the case of a-Si having low carrier-mobility, the size of the TFTs must increase to some extent for providing a certain amount of electric current for driving the pixels. Meanwhile in the case of polysilicon having high carrier-mobility, the size of the TFTs can be reduced. Practically, the pixel pitch is limited to approximately 100 .mu.m in the case of a-Si, while a small pixel pitch of 20 .mu.m can be employed in the case of polysilicon.
With such a reduction in the pixel area, the collecting diameter of the micro-lenses is required to be correspondingly smaller. Although, it is ideal that the light beams entering the micro-lenses from the projection optical system are completely parallel to the optical axis, in practice, the light beams are shifted from the parallel state by a small angle. Thus, light beams which should enter only one pixel reach the opaque regions between adjacent pixels, thereby reducing transmission efficiency. Consequently, the luminance of the display image decreases and the effects of the micro-lenses decline. Additionally, when a light beam, which should enters only one pixel for a certain color (e. g., the pixel for G), enters an adjacent pixel for another color (e. g., the pixel for R), so-called color mixing occurs and deteriorates the color image quality. For example, the incident light intensity and the shift between the original incident angle and the light beam entering a pixel of a certain color (e. g., the pixel for G) has the relationship shown in FIG. 5. The incident light intensity reaches its maximum value at angles slightly shifted from the original incident angle, as is shown in FIG. 5. Therefore, it is understood that the shift of the incident angle greatly affects the luminance and color mixing of the displayed images.
The larger the distance between each pixel portion and the corresponding micro-lens, the more significant the trend becomes. Therefore, with an increasing demand for high resolution, the distance between each pixel and the corresponding micro-lens must be correspondingly reduced. For achieving the above, the following methods can be employed: a method for reducing the focal length by decreasing the size of the micro-lenses while retaining their shape, and a method for reducing the focal length alone without changing the size of the micro-lenses. According to the former method, the aperture angle (the angle subtended by the lens diameter at the focal point) in the light-emerging side does not alter because the shape of the micro-lenses is retained. However, to design and produce such fine micro-lenses is not easy. Furthermore, it is not practical to reduce the size of the micro-lenses while retaining the shape, considering the relationship between the display-element format and other optical parts. Therefore, the focal length is required to decrease without greatly changing the lens diameter.
In such a case, the aperture angle can be raised by positioning the focal points at the pixel portion, as is mentioned above. The divergence angle of the emerging light beams from the micro-lens is thereby enlarged. Thus, to effectively utilize the entire light beams emerging from the liquid crystal panel without eclipsing, the F number of the projection lens positioned behind the liquid crystal panel must be considerably reduced, in other words, it is necessary to employ a bright lens system. This is because eclipses of light beams result in luminance or chrominance non-uniformity in images projected on a screen. However, it is generally difficult and costly to design and produce a lens system having a low F number, and thus the cost for the device as a whole increases.